Leaky mode solar receiver using continuous wedge lens

ABSTRACT

A leaky travelling wave array of optical elements provide a solar wavelength rectenna.

RELATED APPLICATION(S)

This application claims the benefit of U.S. Provisional Application No.61/441,720, filed on Feb. 11, 2011 and U.S. Provisional Application No.61/540,730, filed on Sep. 29, 2011.

The entire teachings of the above application(s) are incorporated hereinby reference.

BACKGROUND

Energy consumption has increased by 50% over the last 25 years with 80%of the world's power still generated by fossil fuels. Clean, renewablealternative energy sources are needed to meet increasing energyconsumption. Photovoltaic solar technology is viewed as one possiblesolution, but to meet the increasing demands, dramatic improvements areneeded in solar conversion efficiency and cost/complexity reduction.Currently available mass produced low cost solar panels are at bestabout 10% efficient, with an upper theoretical limit of about 30%efficiency.

An alternative approach to photovoltaic technology is the fabrication ofan array of optical antennas with rectifier elements located at theoutputs of the optical antennas. These antennas elements are known as“rectennas”. There is no fundamental efficiency limit to rectenna arraysand conversion efficiencies of >85% have been achieved at microwavefrequencies as a method of wireless power transmission. The main reasonthat solar rectenna technology has not achieved wide acceptance is thedifficulty of fabricating efficient rectifier elements at opticalfrequencies that work at the low fields present at each rectennaelement.

SUMMARY

In one implementation, a solar energy apparatus includes a waveguidehaving a top surface, a bottom surface, a collection end and a detectionend. One or more scattering features are disposed on the top surface ofthe waveguide or within the waveguide. The scattering features achieveoperation in a leaky propagation mode.

One or more solar energy detectors are disposed adjacent to thedetection end. The detector may be one or more photovoltaics or aMIM-diode.

The scattering features may take various forms. They may, for example,be a metal structure such as a rod formed on or in the waveguide. Inother embodiments the scattering features may be one or more rectangularslots formed on or in the waveguide. In other embodiments the scatteringfeatures may be grooves formed in the top surface of the waveguide. Theslot and/or grooves may have various shapes.

The waveguide may be a dielectric material such as silicon nitride,silicon dioxide, magnesium fluoride, titanium dioxide or other materialssuitable for leaky wave mode propagation at solar wavelengths.

The scattering feature dimensions and spacing may vary with theirrespective position along the waveguide. For example, adjusting thespacing of the scattering features may assist with the leaky modecoupling to waves propagating within the waveguide, allowing thewaveguide to leak a portion of power along the its entire length, andimproving efficiency or bandwidth.

In other embodiments, selected scattering features may be positionedorthogonally with respect to one another. This permits the apparatus tocapture multiple light polarizations. In this arrangement, thescattering features can be located at each element position in an arrayof scattering features or may be arranged as a set of one-dimensionalline arrays with the features of alternating line arrays providingdifferent polarizations.

In still other arrangements, a wavelength correction element adds lineardelay to incident solar energy received by the waveguide. This permits aresulting beam direction of the apparatus to be independent of thereceive energy wavelength. This correction element may be formed from aset of discrete features embedded in the waveguide with a periodicallymodulated spacing; or it may be embodied as a material layer that tapersfrom a thin section at the collection end to a thick section near thedetection end. The wedge is preferably formed of a material having ahigher dielectric constant then the waveguide. With this arrangement asecond wedge of material may also be provided to compensate fordispersion introduced by the first wedge.

The leaky propagation mode of operation may be further enhanced by acoupling layer placed between the waveguide and a continuous wedge lens.With this arrangement the coupling layer has a dielectric constant thatchanges from the detection end to the collection end, thereforeproviding increased coupling between the waveguide and the wedge lens asa function of the distance along the main axis of the waveguide. Thisfunction may also be provided by a coupling layer decreasing inthickness with distance from the detection end. Such a coupling layermay equalize the horizontal and vertical mode propagation velocities inthe waveguide.

In still other arrangements, the waveguide may itself be formed of twoor more layers. Adjacent layers may be formed of materials withdifferent dielectric constants. Gaps may be formed between the layerswith a control element provided to adjust a size of the gaps. The gapspacing control element may be, for example, a piezoelectric,electroactive material or a mechanical position control. Such gaps mayfurther control the beamwidth and direction.

In still other arrangements, a multilayer waveguide may providefrequency selective surfaces to assist with maintaining a constant beamshape over a range of wavelengths. The spacing in such an arrangementbetween the layers may follow a chirp relationship.

In yet another arrangement, a layer disposed adjacent the waveguide mayprovide quadratic phase weighting along a primary waveguide axis. Thismay further assist in maintaining a constant width solar beam. Thequadratic phase weight may be imposed by a layer having a thickness thattapers from a detection end to a collection end, or may be provided inother ways such as by subsurface elements formed within the waveguidethat vary in length, spacing and/or depth from the surface.

In a preferred configuration, the energy collectors may be formed ofmultiple, spatially separated photovoltaics of different wavelengths.This permits greater range of wavelength operation and therefore,greater efficiency. Anti-reflective coatings may be applied between thesurface of the waveguide and the collectors to further provide forgreater efficiency.

A waveguide having scattering features as described herein can focus areceive solar energy beam along an azimuthal direction. This allows theapparatus to be physically scanned in an elevational direction thatchanges position with time of day, enabling tracking of the sun as itmoves along the horizon.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will be apparent from the following more particulardescription of example embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views.

The drawings are not necessarily to scale, emphasis instead being placedupon illustrating embodiments of the present invention.

FIG. 1-1 is a conceptual diagram of one implementation of a solar arrayusing discrete scattering elements to operate in a leaky propagationmode.

FIG. 1-2 is an implementation using orthogonal scattering elements.

FIG. 1-3 is a specific embodiment as a single dielectric rod with V- andH-polarized scattering features.

FIG. 1-4 is another implementation where the leaky propagation mode isprovided by a continuous structure.

FIG. 1-5 illustrates the H- and V-beam patterns for the implementationof FIG. 1-4.

FIG. 2-1 is a slab embodiment with a group of line arrays havingco-located cross-polarized scattering features.

FIG. 2-2 is a slab embodiment with a group of line arrays havingalternating cross-polarized scattering features.

FIG. 3-1 is a detailed view of a dielectric waveguide with surfacerectangular grooves that provide good single polarization efficiency.

FIG. 3-2 is another embodiment with a dielectric waveguide with surfacetriangular grooves provide good single polarization efficiency.

FIG. 3-3 illustrates metal strips in a cross configuration, offset fromthe centerline to provide co-located features to achieve V and Hpolarization.

FIG. 3-4 illustrates dielectric grooves in a cross configuration alsoproviding collocated V and H polarization response.

FIG. 3-5 shows an implementation that increases the H-pol efficiency(and hence improving the axial ratio) by asymmetrically grooving the Hportion of the element deeper into the waveguide, which also increasesthe coupling for the H pol portion.

FIG. 3-6 separates the V and H pol grooves along the waveguide surface,which further increases radiation efficiency from each scatteringelement because it minimizes cross coupling between adjacent pairs.

FIG. 3-7 shows vertically separate V and H pol elements, which canprovide increased efficiency over collocated “crosses”; while the V andH elements are not technically collocated here, separating thesevertically allows the V and H pol elements to use the same sun-facingsurface area.

FIG. 3-8 illustrates offset spherical lens elements providing both V andH-pol response.

FIG. 3-9 shows how triangular grooves can be combined and collocated fortwo adjacent multi-polarized line arrays in a single sub-array.

FIG. 3-10 is an implementation where the scattering features obtaincircular polarization with interleaved metal strips.

FIG. 3-11 implements metal strips imprinted as dielectric triangular orrectangular grooves to provide V and H pol response.

FIG. 3-12 rotates the orientation of the triangular or rectangulargrooves to provide a mixed V and H pol response.

FIG. 3-13 has scattering features implemented as raised trianglestructures to provide a single polarization response.

FIG. 3-14 is a similar implementation using raised right angle trapezoidstructures to also provide a single polarization response.

FIG. 3-15 shows raised interleaved crosses to provide V and H polresponse.

FIG. 3-16 is an implementation with offset longitudinal slots providingH pol response along the long axis.

FIG. 4-1 illustrates a correction wedge used on the incoming side of arod-type linear waveguide to provide linear delay to the scatteringfeatures.

FIG. 4-2 illustrates the wedge with low dielectric constant gap toimprove performance.

FIG. 4-3 is an alternate embodiment where a surface structure can alsoprovide linear delay.

FIG. 5 is a double wedge to control the dispersion.

FIG. 6 shows the double wedge used with the slab array embodiment.

FIG. 7 illustrates dispersion for various lengths of a dielectric rod.

FIG. 8 shows a waveguide formed of multiple layers having a chirpedspacing to provide frequency selective surfaces (FSS).

FIG. 9 is a more detailed view of the waveguide having surfacescattering features and chirped Bragg FSS layers.

FIG. 10-1 is a tapered dielectric layer to provide quadratic phaseweighting.

FIG. 10-2 is another way to achieve quadratic phase weighting.

FIG. 11 is a way to provide effective dielectric constant control bychanging gap size.

FIG. 12 is a wideband/scanning configuration.

FIG. 13 shows reconfigurable chirped Bragg structures.

FIG. 14 shows a single elongated gap used with multiple arrays.

FIG. 15 is a rod array geometry where the collector region has two ormore spatially separated photovoltaic (PV) cells and anti-reflectivecoatings.

FIG. 16 illustrates a corner reflector in more detail permittingpositioning of the PV cells on both sides of the collector region.

FIG. 17 is a chart illustrating different solar energy spectral bandsand appropriate PV materials for each band.

FIG. 18 illustrates the efficiency of different types of solar cells

FIG. 19 is a dielectric rod with a MIM diode type detector.

FIG. 20 shows a MIM device in more detail.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A description of example embodiments follows.

Traveling Leaky Wave Array

Recent attempts at creating a rectenna array have resulted inefficiencies ranging from 1% to 2%. Each antenna in the array has itsown rectification element, leading to very low fields generated acrossthe rectifier.

In preferred embodiments herein, much improved efficiency is provided bya waveguide structure having scattering features arranged in one or moresubarrays. The subarrays operate in a leaky propagation mode atwavelengths ranging from about 0.4 to 2.0 microns. Placement ofdetection element(s) across the subarray outputs increases the receivedsolar energy field by orders of magnitude over that of a single element.

In particular embodiments, the subarray makes use of beam formingtechniques relying on 1) quadratic time delay feeds to maintain thebeamwidth at a constant width (e.g., 0.5 degrees, the solar angularwidth,) and 2) changing the direction of the beam by adjusting thepropagation constant of the array. The multiple parallel 1-D subarraysprovide fan beams which can track the sun as it progresses from east towest.

Single line source leaky wave antennas can be used to synthesizefrequency scanning beams. The array elements are excited by a travelingwave progressing along the array line. Assuming constant phaseprogression and constant excitation amplitude, the direction of the beamis that of Equation (1).

Cos θ=β(line)/β−(λm)/s  (1)

where s is the spacing between elements, m is the order of the beam, β(line) is the leaky mode propagation constant, and β is the free spacepropagation constant, and λ is the wavelength. Note the frequencydependence of the direction of the beam.

To minimize losses, an array of dielectric surface wave guides feedsemploys an array of one-dimensional, dielectric rectangular crosssection subarray waveguides (also called “rods” herein). Alternately,one large panel or “slab” of dielectric can house multiple linesubarrays.

Treating each of the subarrays as a transmitting case, the dielectricstructure is excited at one end and the energy travels along thewaveguide. The waveguide leaks and scatters a small amount of the energyalong its length until at the end whatever power is left is absorbed bythe resistive loads. In the far-field, the scattered energy from eachelement will be in phase, creating a high gain beam at some angle.

More particularly, treating each of the sub arrays as a solar energyreceiver, the dielectric structure receives solar energy at a collectorend, and the energy travels along the waveguide. The waveguide absorbsand scatters a small amount of the energy along its length until all thein-phase components reinforce at the detection end, where a solarcollector (e.g., a photovoltaic or MIM diode, etc.) absorbs the solarenergy converting it to electrical energy.

In one embodiment, dielectric surface waveguide feeds employ an array ofdielectric rectangular cross sectional rods as the primary elements.

One preferred geometry is generally shown in FIG. 1-1. A set ofdielectric rods 100 are disposed in parallel on a substrate 200 andextend from a collector end 250 to a detection end 260. Each dielectricrod provides a single one-dimensional sub array. Scattering elements 400disposed along each of the rods are provided by conductive strips formedon, groves cut in the surface of, or groves entirely embedded into, thedielectric. Each rod may be as long as 100 solar wavelengths, and theremay be many such scattering elements, as many as 10,000 elements, alongeach rod.

The HE11 mode, which has no cut off frequency, is the propagating modeof choice for the waveguide.

As generally shown in FIG. 1-2, adjacent rods 100-01, 100-2 may havescattering features 400-1, 400-2 with alternate orientation(s) toprovide orthogonal polarization (such as at 90 degrees to provide bothhorizontal (H) and vertical (V) polarization). This tends to maximizeenergy capture since solar energy is unpolarized (randomizedpolarization).

Construction

As mentioned briefly above, these sub arrays can be constructedindividually, such as in a linear one-dimensional rod configuration (seeFIG. 1-3 for an example) or with multiple line arrays located on orwithin a single dielectric panel or “slab” (see FIGS. 2-1 and 2-2 forexamples).

Individual scattering element design is dependent on the choice ofconstruction and will be described in more detail below. It sufficeshere to say that the scattering elements and can be provided in a numberof ways, such as conducting strips or non-conducting grooves embeddedinto the dielectric waveguide.

Collocated elliptically polarized elements provide polarizationdiversity to maximize the energy captured since solar energy is randomlypolarized. In one embodiment, that shown in FIG. 1-3, surface groves 105are co-located and orthogonally disposed with respect to embedded areascut-out 107 of the dielectric at each position in the array. In thisimplementation, the width of the grove 105 in the upper surface of thewaveguide 100 may change with position along the waveguide. If λ is thewavelength of operation of the sub array, the grove width may incrementgradually, such as from λ/100 at the detection end 250 to λ/2 at thecollection end 260; the spacing between features may be constant, forexample, λ/4.

FIG. 1-4 illustrates another way to implement leaky mode operation.

Individual scattering elements originally embedded in or on thewaveguide can be replaced with a continuous leaky wave wedge structure175. The coupling between the waveguide and the wedge preferablyincreases as a function of distance along the waveguide to facilitateconstant amplitude along the radiation wavefront. This may beaccomplished by inserting a third layer 190 between the wedge and thewaveguide with a decreasing thickness along the waveguide. This couplinglayer 190, preferably formed of a material with yet another relativepermittivity constant, ensures that the power leaked remains uniformalong the length of the corresponding rod or slab.

The propagation constant in this “leaky wedge with waveguide”implementation of FIG. 1-4 determines the beam direction. To receiveboth horizontal and vertical polarization at a given beam direction, thepropagation constants for horizontal and vertical modes of thewaveguide-wedge configuration must be equal. There is a slightdifference in the propagation constants for the H- and V-pol modes,which is manifested as a slight difference in the beam direction (3degrees) as shown in FIG. 1-5. The vertical beam is shifted more thanthe horizontal implying a slightly higher propagation constant. Byapplying a thin layer of high dielectric material on the bottom of thewaveguide, the horizontal propagation constant can be increased relativeto the vertical resulting in the beams coinciding.

Slab Configuration

As mentioned briefly above, groups of sub arrays can be disposed on asubstrate formed as a two-dimensional panel or slab 300. In these slabconfigurations, the sub arrays are orthogonally polarized to achievehorizontal (H) and vertical (V) polarization, either with collocatedcross-polarized scattering features (such as in the FIG. 2-1configuration), or alternating sub-arrays of cross-polarized scatteringfeatures (FIG. 2-2). It is recognized that if collocated orthogonallypolarized features are as efficient as a single polarization embodiment,the overall efficiency of the device will be greater by utilizing moresun-facing surface area with both polarizations.

The waveguide in these slab configurations operates in a TM and TE modein the vertical and horizontal.

The FIG. 2-1 and FIG. 2-2 slab configurations may be formed on a siliconsubstrate (not shown) with the dielectric waveguide embodied as a set ofwaveguide core sections, including (a) a main core section 401 startingadjacent the collection end and extending to (b) a tapered section 402and (c) a lossy core section 403 extending from the tapered section tothe detection end 260. Suitable dielectric materials include Si₃N₄,SiO₂, MgF₂, and TiO₂.

A cladding layer (not shown) may be disposed between the main waveguidesection and/or tapered core section(s).

The lossy waveguide section disposed adjacent the detection end housesmultiple detectors of different wavelengths as described in more detailbelow.

This slab implementation can provide ease of manufacture and betterperformance by eliminating edge effects.

Scattering Feature (Element) Designs

There are a multitude of element makeups that provide varying degrees ofefficiency. Due to metal Ohmic heating losses and manufacturability atthese sizes, it is desirable to use a dielectric groove or imprintstructure. However, it is also possible to use metalized elements tocapture the same effect, albeit with higher losses. The followingfigures show element shapes that have varying degrees of ellipticity,and/or high efficiency in a single polarization. With all element cases,there remain two similarities. The element spacing distribution has aneffect on the frequency of operation and bandwidth of the array. Foreach element type and bandwidth desired, the spacing of element toelement is optimized. For most element types, there is a widthdistribution increasing along the long axis of the sub-array, asmentioned above. The intention of this increasing width distribution isto couple and scatter a similar amount of energy from each element. Todo this, the elements near the detector region (or feed) are narrower,so they do not scatter as much energy per unit area as the elementsfurther down the long axis. The width distribution is adapted forexample, from Rodenbeck, Christopher T., “A novel millimeter-wavebeam-steering technique using a dielectric-image-line-fed grating film”,Texas A & M University, 2001 at equation 3. This width relationship isoptimized for each element type to maximize array radiation efficiency.

FIGS. 3-1 though 3-16 depict various scattering element shapes for boththe one-dimensional rod and array (slab) configurations.

FIG. 3-1 is a single rectangular dielectric rod waveguide 160 withsurface rectangular grooves 150 that provide single polarization.

FIG. 3-2 is another embodiment with a dielectric rod waveguide 100 withsurface features shaped as triangular grooves 151.

FIG. 3-3 illustrates metal strips 501 disposed on the surface of thedielectric rod 100. The strips are shaped in a cross configuration, andare preferably offset from a centerline of the rod. This arrangementprovide co-located features to achieve V polarization (V-pol) and Hpolarization (H-pol).

FIG. 3-4 illustrates dielectric grooves 502 in a cross configurationalso providing collocated V and H polarization response.

FIG. 3-5 shows an implementation that increases the H-pol efficiency(and hence improving the axial ratio) by asymmetrically grooving the Hportion 570 of the element deeper into the waveguide, which alsoincreases the coupling for the H-pol portion.

FIG. 3-6 separates the V-pol and H-pol 580, 581 grooves along thewaveguide 100 surface, which further increases radiation efficiency fromeach scattering element because it minimizes cross coupling betweenadjacent pairs.

FIG. 3-7 shows vertically separate V- and H-pol elements 590, 591, whichcan provide increased efficiency over collocated “crosses”. While the V-and H-pol elements are not technically collocated here, separating thesevertically allows the V- and H-pol elements to use the same sun-facingsurface area.

FIG. 3-8 illustrates arrays of offset spherical lens elements 594-1,594-2 providing both V and H-pol response.

FIG. 3-9 is an implementation using triangular grooves that can becombined and collocated for two adjacent multi-polarized line arrays ina single sub-array. Note that the width of the grooves 600 changes withposition along the sub-array.

FIG. 3-10 is an implementation where the scattering features obtaincircular polarization with interleaved metal strips 610.

FIG. 3-11 implements metal strips imprinted as dielectric triangular orrectangular grooves 620, 621 to provide V and H-pol response.

FIG. 3-12 rotates the orientation of the triangular or rectangulargrooves 630 to provide a mixed V and H pol response.

FIG. 3-13 has scattering features implemented as raised trianglestructures 640 to provide a single polarization response.

FIG. 3-14 is a similar implementation using raised right angle trapezoidstructures 641 to also provide a single polarization response.

FIG. 3-15 shows raised interleaved crosses 650 to provide V- and H-polresponse.

FIG. 3-16 is an implementation with offset longitudinal slots 670, 671providing H-pol response along the long axis.

Wideband Operation

A significant challenge is the instantaneous bandwidth of the array.Equation (1) indicates that there is a shift in the beam direction asthe frequency changes. This distortion is caused by the fact that allusable beams are higher order beams. Since we want the cover the bandbetween 0.4 to 2.0 microns, we have developed two methods of correctingthis frequency distortion. (Please note that other bands are possible).

Correction Wedge

FIG. 4-1 shows a dielectric rod type one-dimensional (1-D) sub-array 305configuration with surface scattering features similar to that of FIG.3-1 decreasing in size with position from the collection end to thedetection end.

The first approach to correcting frequency distortion introduced by thisgeometry is to situate a correcting layer 700 on top of the sub-array305. This layer, shown in FIG. 4-1, permits the use of the principal m=0order.

The idea behind the correction layer, is to linearly add increasingdelay to the scattering elements from the collector end to the detectionend. Incident radiation enters along the top surface of the wedge and isdelayed depending upon the location of incidence. When this is doneproperly, the quiescent delay for each element across the top plane ofthe correction layer is therefore the same, regardless of the positionalong the sub-array at which the solar energy was received. The effectis that in the far-field, the beams over frequency line up at the samepoint, which resulting beam would then be aimed at the sun.

One implementation that has been modeled indicates a TiO₂ top wedgelayer 700, and a lower dielectric SiO₂ waveguide 100. Forming thecorrection wedge of a higher dielectric permits it to be “shorter” inheight”. There are a multitude of materials that can be used toimplement the correction wedge 700. The propagation constant of thewaveguide should also be constant as a function of frequency, which isachieved by operating in the constant propagation regions of thewaveguide as shown in FIG. 7 (the waveguide dispersion curves).

Linear delay can be implemented in other ways. For a multiple rodimplementation, depositing a set of wedges, like wedge 700 for each 1-Darray would be tedious. Instead, one can fabricate a molded plasticsheet with a series of wedges. In other implementations, a TiO₂ layerwith top facing groves can replace the wedge to re-radiate the energyincident on the scattering elements as per FIG. 4-3. A coupling layerwith a tapered shape but constant dielectric may be disposed between theTiO₂ and SiO₂ layers.

Since the wedge of FIG. 4-1 may introduce unwanted dispersion along thearray, it may be necessary to compensate. There are two options.

1) It is possible to insert a low dielectric constant gap 782 (FIG. 4-2)between the wedge and the dielectric waveguide 100. This gap 782 allowsthe waveguide to guide the wave while not affecting the propagationconstant. The wedge 700 sitting above this gap still retain its delaycharacteristics for each element of the 1-D array.

2) A second wedge 790 can be situated beneath the waveguide 100, holdingconstant the dispersion along the waveguide (FIG. 5). The same techniquecan be used on the slab line arrays (FIG. 6).

Chirped Bragg Layers

Chirped Bragg layers situated underneath the waveguide structure canalter the propagation constant of the waveguide as a function offrequency. In this way, it is possible to line up beams in thefar-field, making this antenna broadband.

The dispersion of the dielectric rod is shown in FIG. 7 for variousdiameters (D) of the rod. F_(c) is the center frequency of the desiredband (F_(u)−F₁). As the diameter changes from 0.1 wavelengths to 0.4wavelengths, C/V, the ratio of free space velocity to velocity in therod, increases along the rod.

An embodiment of an apparatus using such Frequency Selective Surfaces(FSS) shown in FIG. 8. These FSS, also known as chirped Bragg layers,are provided by a set of fixed layers of low dielectric constantmaterial 1012 alternated with high dielectric constant material 1010.The spacing of the layers is such that the energy is reflected where thespacing is ¼ wavelength. The relatively higher frequencies (lowerwavelengths) are reflected at layers P1 (those nearer the top surface ofwaveguide 100) and the lower frequencies (high wavelengths) at layers P2(those nearer the bottom surface). The local (or specific) layer spacingas function of distance along P1 to P2 is adjusted to obtain therequired propagation constant as a function of frequency to achievewideband frequency independent beams. Equation (1) can be solved for agiven beam direction to obtain the geometry of the chirped Bragg layers.

FIG. 9 is a depiction of the waveguide with multiple chirped Bragglayers 1010, 1012 located beneath a primary, non-Bragg waveguide layer1030. This example (the illustrated Bragg layers are not to scale) wasmodeled using alternating layers made up of SiO2 and TiO2; however anymaterial(s) with differing dielectric constants could be used in theselayers.

Spacing of the Bragg layers 1010, 1012 can be determined as follows. Anequation governing the beam angle of a traveling wave fed linear arrayis:

cos(theta)=beta(waveguide)/beta(air)+lambda/element spacing

where beta (waveguide) is the propagation constant of the guide.

To eliminate the frequency dependency of theta, we solve the equationfor beta (waveguide). The required frequency dependency of beta can befashioned by controlling the effective thickness of the waveguide as afunction of frequency derived by using the general dispersion curve ofthe waveguide itself.

The effective thickness as a function of frequency is then provided by aseries of chirped Bragg layers as shown in FIG. 9 forming the waveguide.Each layer is composed of two sub layers of a high dielectric and a lowdielectric. Each sub layer is preferably ¼ wavelength thick at thefrequency at which energy is reflected in that layer. The layers getprogressively thicker such that the lower frequencies reflect at thethicker layers. The methodology of determining the geometry of thelayered structure is a recursion relation involving creation of theabove layers starting at the top layer (L=1), the reflecting layer atthe highest frequency f(1). The next layer (L=2) is determined by therelation T(f(L))−T(f(L−1))=k/f(L) where k is the average velocity in thestructure, and L is the layer number. The next adjacent layer followsthis recursive relationship, and so forth.

Beamwidth Control

To increase the array length while maintaining a beamwidth, quadraticphase weights may be added. Because the solar angular arc width is 0.5degrees, we preferably maintain a beamwidth around 0.5 degrees acrossall frequencies. This can be accomplished by implementing a quadraticphase weighting along the primary axis of the 1-D array, and can beachieved with either 1) gradually tapering a dielectric layer itself (asshown in FIG. 10-1) located over the scattering elements 400 or 2) asub-surface array of elements with quadratic length taper along thearray axis (FIG. 10-2). The quadratic phase weighs the physical size ofthe array beyond 100 λ.

The sub-surface elements within the waveguide can be varied in length,spacing, and or depth within the waveguide to obtain the desiredquadratic phase weighting. Regardless, the sub surface elements arelocated deep enough within the waveguide so as to not radiate outsidethe waveguide. The tapered layer be defined by

φ(x)=e ^(iαx) ²

where x is the distance along the waveguide and α is a weightingconstant.

Scanning and Steering

The high gain fan beams of the 1D sub array “rods” therefore need onlybe steered in one dimension in order to track the sun as it travelsacross the sky. This steering can be achieved in two ways: mechanicaland electrically. The 1D tracking requirement facilitates eithermechanical or electrical tracking methodologies.

Mechanical

In this approach, the traveling wave solar cell is placed on a supportthat is mechanically positioned utilizing a positioner or some othermechanical means such as MEMs or electro active devices.

Electrical

In this approach, the system electrically scans the main beam bydynamically changing the volume or spacing of gaps in the dielectricwaveguide. It is equivalent to changing the “effective dielectricconstant,” causing more or less delay through the waveguide. The fieldsassociated with the HE11 mode (the mode operating in the rod typewaveguide) are counter propagating waves traversing across the gaps asshown in FIG. 11. The effective dielectric constant change isindependent of frequency as long as the gap spacing, s, is less than ¼wavelength.

Electrical scanning can be achieved by controlling the gap size by withpiezoelectric, electro active, or any other control element that is fastacting to effect a change in the propagation constant of the waveguide.The wedge configuration of FIG. 4 is readily amenable to incorporationof the gaps in the waveguide.

To achieve wideband propagation constant control, an additional chirpedBragg structure can be provided to adjust the effective rod diameter asa function of frequency. FIG. 12 shows this additional feature, chirpedBragg frequency selective surfaces (FSS), added to the structure of FIG.11.

As an added degree of freedom, enhancing the Bragg FSS structure withreconfigurable dielectric layers (FIG. 13) provides better beam steeringprecision and efficiency. By chirping the structure, the widebandproperties of the FSS Bragg layers takes effect, allowing frequencyindependent beams. With this approach, the reconfigurable structure andBragg FSS are one in the same.

Rectification

As will now be understood, the sub-array excites a traveling wave or“net energy flow” in the body of the dielectric waveguide, travelingfrom the collector region towards the detector region at an intensityequal to or greater than 100 suns. The HE11 mode, which has no cut offfrequency, would be the mode of choice. This mode is set up by properlydimensioning the waveguide and a careful selection of scattering elementsize distribution and layout.

FIG. 14 illustrates an arrangement where multiple 1-D arrays 1500-1,1500-2, . . . 1500-n are positioned end to end. An extended correctionwedge 1510 covers the length of the assembly above the 1-D arrays 1500.An angular configurable gap 1550 may be provided to introduce a smallchange in wave propagation and can be controlled mechanically or withapplication of heat. A small charge in this gap angle can result in alarge change in beam direction.

Spatially Separated Photovoltaic Cells

FIG. 15 illustrates one possible configuration of the collectors regionin more detail, consisting of a set of spatially separated photovoltaics(PV) 1600. Each of the PV detectors 1600 has a different effective setof responsive wavelengths (“response band”). In this embodiment, 12detectors (2 for each of six bands from band 1 to band 6) are provided;note that half the detectors can be placed on a top face of thewaveguide and the other half on a bottom face of the waveguide. A cornerreflector (shown in detail in FIG. 16) ensures wave propagation to thedetectors mounted on the bottom face.

A different corresponding tuned anti-reflective (AR) coating 1620 isdisposed between each detector 1600 and the waveguide 100 surface(s), toaccept only in-band solar energy for each PV detector.

With the HE11 hybrid mode operation of the waveguide,counter-propagating waves are incident on the edges of the waveguide.When these waves reach the collector region, they are incident on the ARcoating 1620 and PV cells 1600 at near normal incidence.

The sub array geometry in FIG. 15 shows the collector region where solarenergy is converted to DC electricity. The energy incident on the subarray is transmitted along the dielectric waveguide to the collectorregion.

The photovoltaic cells appropriate for each band (corresponding witheach wavelength region of the solar band as depicted in FIG. 17) linethe waveguide surfaces where collected energy is incident. Since almostall of the solar energy is contained in 6 wavelength bands from 0.4micrometers to 1.75 micrometers, it is sufficient to use the 6 distinctphotovoltaic detectors as were illustrated in FIG. 15. For thesedistinct bands, there exist photovoltaic cells that are very efficientfor their respective bands. While multi-band photovoltaic assembliesexist in previous technology, such assemblies are often not efficientbecause the cells are stacked on top of one another. By spatiallyseparating the bands, we eliminate this problem. In addition, due to thehigh gain nature of the travelling wave array, the energy incident onthe photovoltaic can be as great as 100 suns in intensity, furtherincreasing the efficiency of this technology.

An anti-reflective (AR) coating 1620 with a pass band corresponding toeach band is applied to the waveguide abutting side of the photovoltaiccell 1600. This causes lower frequency energy to travel along thewaveguide undisturbed until it reaches the PV collector regionappropriate for its wavelength.

This spatial separation of differing bands of photovoltaic cells, whilenot sacrificing any sun-facing surface area causes this solar cell toreach much closer to the theoretical efficiency limit than previoustechnology, as shown in FIG. 18. The fact that the rectifying collectorregion surface area is small compared to the energy absorption surfacearea (antenna to collector region surface ratio>100/1) further supportshigh solar conversion efficiency. The rectifying region can also belocated below the sun-facing area (by means of a bend in the waveguideto transfer energy in another direction), freeing up extra area forincreased traveling wave density.

Metal/Insulator/Metal Diodes

The Metal Insulator Metal Diode (MIM) is an emerging technology thatpromises wideband coupled rectification. Work has been done in bringingthis technology to realization—see Sachit Grover, Olga Dmitryeva, M.Estes, Garret Moddel: “Traveling-Wave Metal/Insulator/Metal Diodes forImproved Infrared Bandwidth and Efficiency of Antenna-CoupledRectifiers”, IEEE Transactions Nanotechnology. 9(6), (November 2010).One possible geometry of the preferred MIM device is depicted in FIG.20. These MIMs can be placed at the detection end of the traveling waveantenna waveguide and absorb energy incident on their surface (see FIG.19). Because MIMs are efficient over a wide band (most of the solarspectrum), there is no need to spatially separate multiple such devices,simplifying the build process.

Other Modes of Operation

-   -   Wideband solar with fixed beam (no scanning layers) and        mechanical means of elevation beam scanning with micro        mechanical actuation.    -   Wideband phased array with beam scanning    -   Narrowband beam scanning with no frequency selective layers        needed.

The teachings of all patents, published applications and referencescited herein are incorporated by reference in their entirety.

While this invention has been particularly shown and described withreferences to example embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

1. A solar energy apparatus comprising: a waveguide having a topsurface, a bottom surface, a detection end, and a collection end; acontinuous wedge lens disposed on the waveguide; and a coupling layerdisposed between the waveguide and the continuous wedge lens, to provideoperation in a leaky solar energy receive mode.
 2. The apparatus ofclaim 1 wherein the waveguide operates in both Hand V-pol modes and thewedge is isotropic.
 3. The apparatus of claim 1 wherein the couplinglayer has a dielectric constant that tapers from the detection end tothe collection end.
 4. The apparatus of claim 1 wherein the couplinglayer provides increasing coupling between the waveguide and the wedgelens as a function of distance along a main axis of the waveguide. 5.The apparatus of claim 1 wherein the coupling layer decreases inthickness as distance from the detection end.
 6. The apparatus of claim1 wherein the coupling layer reduces dispersion for incident radiationreceived along an entry surface of the wedge lens.
 7. The apparatus ofclaim 1 wherein the coupling layer equalizes H- and Vmode propagationvelocities within the waveguide.
 8. The apparatus of claim 1 wherein awedge element corrects dispersion introduced by the coupling layer. 9.The apparatus of claim 1 wherein a bottom layer equalizes H-pol andV-pol velocities.
 10. The apparatus of claim 1 wherein the waveguide isa slab.
 11. The apparatus of claim 1 wherein a dispersion correctingwedge is disposed beneath the slab to correct for dispersion introducedby the delay wedge.
 12. The apparatus of claim 1 wherein a quadraticphase height is provided by a layer having a tapered thickness.